1. Field of the Invention
This invention relates to a process for converting molten iron from a blast furnace into molten steel. The process provides an improved refining which comprises a systematic division of the sequence of treating molten iron.
2. Description of the Prior Art
(1) A typical example of the process for converting molten iron from a blast furnace into molten steel is the basic oxygen converter process which comprises blowing oxygen onto the molten iron in the presence of basic slag to achieve simultaneous reduction of the C, Si, P and S contents of the melt to the desired levels. However, the onverter process involves oxidation reaction which performs decarburization, desiliconization and dephosphorization simultaneously in the converter, and accordingly, high bath and atmospheric temperatures are generated. The dephosphorization reaction proceeds at relatively low temperatures, and to accomplish efficient dephosphorization, the slag formation must be controlled while its basicity is held high. However, due to desiliconization, silicon is oxidized earliest into silicic anhydride (SiO.sub.2) which thereby reduces the basicity of the slag and inhibits dephosphorization. Therefore, to achieve the proper control of the slag basicity, much flux such as CaO is needed, and this results in the formation of as much as 120 to 150 kg/t of slag. Steel making operations in the presence of much slag often cause slag foaming or slopping, and to prevent such unwanted phenomena, a large-capacity converter must be used, resulting in an increase in the cost of the stell mill. Besides, the discharge of much slag increases the load and operating cost of a recovery or regenerating system, and in addition, the limited use of slag makes a large slag dumping yard necessary. The formation of much slag also means low iron yield because slag contains about 20% of FeO. (This includes a little Fe.sub.2 O.sub.3). What is more, the high slag content causes early damages to the furnace refractory and complicates the converter operation, causing various problems such as low quality molten steel due to its absorption of hydrogen from the flux and to its increased oxygen content as well as the need of adition of ferroally and low steel yield.
(2) The process of performing desulfurization and dephosphorization in two separate stages has been proposed in, for example, Japanese Patent Publication No. 42696/71. The basic concept of the process is to remove impurity elements such as Si, P and S prior to decarburization by blowing oxygen gas. However, the prior art reference does not teach desiliconization effected before desulfurization and dephosphorization. Instead, the reference indicates simultaneous occurence of desiliconization with dephosphorization which follows desulfurization and is performed in the presence of an oxidizer. Dephosphorization requires the basicity to be held at a predetermined level, but since the molten iron is yet to be desiliconized, all of the silica source is carried over into the dephosphorization step, thus requiring a sufficient amount of flux to achieve the desired silica removal, and therefore, the amount of slag formed in the process is not appreciably smaller than that formed in the converter process. In addition, the question arises as to which treatment should be combined with which (e.g. desiliconization-desulfurization, or dephosphorization-decarburization). One of the critical factors to be considered in solution of the question is that of heat balance. How can a high level of C content in the molten iron be maintained in the treatment that precedes decarburization if it is to serve as a heat source in decarburization? Because of these yet to be solve problems, the described process of performing desulfurization and dephosphorization separately is not being operated on an industrial scale.